cara eject usb di terminal linux

sudo eject /dev/sda

Other answers here that indicate that you require mechanical ejection hardware are incorrect.
Unmounting is not the same thing as ejecting.

1. If you unmount a volume, you can immediately mount it back, because the underlying device is still available. In some situations, this could present a security risk. By ejecting the device, only a reset of the USB subsystem (e.g. a reboot) will reload the device.
2. By ejecting the device, you effectively disable any further access to the device. Only a reset of the USB subsystem (e.g. a reboot) will reload the device. Otherwise, you must physically disconnect the USB device and reconnect it in order to access it again.
3. Before ejecting, this command will unmount all volumes on the device that were mounted.
4. If volumes are in use, this command will fail as with unmount, except that some volumes might be unmounted and some volumes might remain mounted.